Truss Design

[strguy11]

When designing a steel truss, is it common practice to assume the chords are braced laterally, in the weak direction (out of plane) at the panel points, even if there are no transverse members that frame into the panel?

 

[oneintheeye]

they are braced laterally if they are braced. If they are not, they are not?

 

[pelotoner]

No, that is why we use bridging (bar joists) and rat-runs (trusses). And the bracing cannot just run perpendicular between trusses but to another fixed member, many times a diaphragm. On truss bridges (ie warren truss, etc. with 2 main truss members), diagonal members are placed between the compression chords to transfer the bracing into axial forces into the chords, thus providing bracing.

 

[Castigliano]

Generally, if there is nothing framing into the panel point, there is no bracing. The exception being Pony trusses, which use a frame consisting of the floorbeam (in a bridge truss) and the truss verticals to brace the top flange.

Castigliano

 

[Lion06]

In a typical truss, with the top chord loaded, not only are the panel points NOT a brace point for the bottom chord, but the panel points are what causes the need for braces. The compression diagonals need to be braced by the top and bottom chords. As someone already noted, the diaphragm usually provides this at the top chord, but you definitely need some bracing for the bottom chord.
Just be sure to check the strength and stiffness per AISC.

 

[MikeE55]

Castigliano, Would you mind explaining how to design the pony truss concept. Do you use a moment connection at the truss verticals and bridge floor beams to develop the rigidity needed to prevent the top chord of the truss from buckling? How much lateral deflection is acceptable?

 

[bridgebuster]

MikeE55

If you have a copy of the AASHTO Guide Spec for pedestrian bridges they provide an example.

Here's what the AASHTO "Guide Specifcations for Design of Pedestrian Bridges" says:

1.3.6 Half-Through Truss Spans

1.3.6.1 The vertical truss members and the floorbeams and their connections in half-through truss spans shall be proportioned to resist a lateral force applied at the top of the truss verticals that is not less than 0.01K times the average design compressive force in the two adjacent top chord members where K is the design effective length factor for the individual top chord members supported between the truss verticals. In no case shall the value for 0.01K be less than 0.003 when determining the minimum lateral force, regardless of the K-value used to determine the compressive capacity of the top chord. This lateral force shall be applied concurrently with these members’ primary forces.

End posts shall be designed as a simple cantilever to cany its applied axial load combined with a lateral load of 1.0% of the axial load, applied at the upper end.

1.3.6.2 The top chord shall be considered as a column with elastic lateral supports at the panel points. The critical buckling force of the column so determined shall be based on using not less than 2.0 times the maximum design group loading in any panel in the top chord.*

*For a discussion of half-through truss designs, refer to Galambos, Guide to stability Design Criteria for Metal Structures, 4th ed., l9û8, New York: John Wdey and Sons,
Inc., pp. 515-529.

Commentary
1.3.6 Half-Through Truss Spans

This article modifies the provisions of AASHTO Article 10.16.12.1 by replacing the 300 pounds per linear foot design requirement for truss verticals with provisions based on research by Holt and others. These provisions establish the minimum lateral strength of the verticals based on the degree of elastic lateral support necessary for the top chord to resist the maximum design compressive force. The use of 2.0 times the maximum top chord design load to determine the critical buckling force in the top chord is in recognition that under maximum uniform loads, maximum compressive stresses in the top chord may
occur simultaneously over many consecutive panels.

 

[MikeE55]

Thanks Bridgebuster.

 

[nuche1973]

It's been my experience that the top and bottom chords are not assumed to be braced laterally. This is one of those items that must be checked. Also, you'll need to check the part of the structure you are bracing to.

 

[civeng80]

Can anyone tell me what a pony truss is and how it differs from an ordinary truss?

Also how is it ananlysed and designed?

 

[Castigliano]

Bridgebuster's post is a good starting point for pony truss info. If you want to learn more, look in the "Guide to stability Design Criteria for Metal Structures," the latest by Galambos (4th edition, I think). This has quite a bit of information and explanation on pony truss behavior and design.

Castigliano